Science Fair Project Encyclopedia
An equal-loudness contour, also called a loudness level contour or a Fletcher-Munson curve, is a measure of sound pressure (dB SPL) vs. frequency for which a listener perceives a constant loudness. The unit of measurement for loudness levels is the phon, and by definition two sine waves that have equal phons are equally loud.
The humans auditory system is sensitive to frequencies from 20 Hz to a maximum of around 20,000 Hz, although the hearing range decreases with age. Within this range, the human ear is most sensitive between 1 and 5 kHz, largely due to the resonance of the ear canal and the transfer function of the ossicles of the middle ear.
Equal-loudness contours were first measured by Fletcher and Munson using headphones. In their study, listeners were presented with pure tones at various frequencies and over 10 dB increments in stimulus intensity. For each frequency and intensity, the listener was also presented with a reference tone at 1000 Hz. The reference tone was adjusted until it was perceived to be of the same loudness as the test tone. Loudness, being a psychological quantity, is difficult to measure, so Fletcher and Munson averaged their results over many test subjects to derive reasonable averages. Headphones change the acoustics of the ear, however (particularly attenuating low frequencies), and more recent studies have been measured in anechoic chambers and these refined loudness curves are now standardised by ISO.
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